1. Field of the Invention
The present invention relates to the field of reduction of iron- and titania-containing impurities from clays. Specifically, the present invention relates to an improved method of using magnetic separation to remove iron- and titania-containing impurities from clay, and thus to make a high brightness clay.
2. Background Art
Naturally occurring minerals such as natural clays, carbonates, talcs or so forth, frequently include discoloring contaminants in the form of iron and/or iron-stained titanium-based impurities. In many natural clays, including kaolin clays, and particularly the sedimentary kaolins of Georgia, such impurities are commonly present as iron-stained anatase and rutile. For kaolin clays and others of the minerals mentioned, it is often desired and sometimes imperative to refine the natural product and bring the brightness characteristics thereof to a level acceptable for paper coating and other applications. Various techniques have been used previously to effect the removal of this type of discoloring impurity.
For example, hydrosulfites have been widely used for converting at least part of the iron-based (or "ferruginous") impurities to soluble form, which may then be removed from the clays. Additionally, among the most effective methods for removing titaniferous impurities, including, e.g., iron-stained anatase, are the well-known froth flotation techniques. According to such methods, an aqueous suspension or slurry of the clay is formed, the pH of the slurry is raised to an alkaline value, for example, by the addition of ammonium hydroxide, and a collecting agent is added, as for example, oleic acid. The slurry is then conditioned by agitation for a relatively sustained period. A frothing agent, such as pine oil is then added to the conditioned slurry, after which air is passed through the slurry in a froth flotation cell to effect separation of the impurities.
In recent years, it has further been demonstrated that high intensity magnetic separation techniques may be utilized for removing certain of this type of impurity, including titaniferrous impurities, and certain ferruginous matter. Anatase, for example, and certain other paramagnetic minerals, have been found to respond to high intensity magnetic fields.
For example, U.S. Pat. No. 3,471,011 (Lannicelli et al.), discloses that clay slurries may be beneficiated by retention for a period of from about 30 seconds to 8 minutes in a magnetic field of 8,500 Gauss or higher. U.S. Pat. No. 3,676,337 (Kolm) discloses a process for treating mineral slurries by passing same through a steel wool matrix in the presence of a background field of at least 12,000 Gauss. Various apparatus, such as that disclosed in Marston, U.S. Pat. No. 3,627,678, may be utilized in carrying out the Kolin processes. In this latter instance the slurry is thus passed through a canister, which contains a stainless steel or similar filamentary ferromagnetic matrix, while a high intensity magnetic field is impressed on the matrix by enveloping coils.
Magnetic separation can be an inexpensive, selective and efficient method for separating a particulate mixture. Many techniques have been used to magnetize nonmagnetic or paramagnetic particles to permit them to be selectively separated from a mixture by magnetic separation. For example, Shubert U.S. Pat. No. 3,926,789 discloses wetting the surface of mineral particles with an emulsified magnetic fluid to render them magnetic. C. de Latour, Journal of Americain Waterworks Association, Vol. 68, 443 (1976) discloses using an inorganic coagulant, such as ferric chloride or aluminum sulfate, to agglomerate particles nonselectively in a system which contains a mixture of magnetite and other materials. J. Y. Hwang, et al., IEEE transactions on Magnetics, Vol. MAG-18, No. 6, p. 1689 (1982) discloses adding an organic polymer flocculent to a mixture of magnetite and other minerals to yield a selective co-flocculation of magnetite and desired minerals. Yen, et al., U.S. Pat. No. 4,285,819 and Whitehead, et al., U.S. Pat. No. 4,554,088 disclose methods which involve coating magnetic materials with a polymer and then coupling the polymer-coated magnetite particles to the particles to be magnetized. P. Parsonage P, IMM Tenth Annual Commodity, Paper No. W86007 (1985) discloses introducing fine magnetite into a pulp of mineral slurries in which the desired minerals are conditioned to carry a surface charge opposite to that of magnetite to favor coating of magnetite. Briggs, et al., U.S. Pat. No. 4,019,995, Shimoiizaka U.S. Pat. No. 4,094,804, Khalafalla, et al., U.S. Pat. No. 4,208,294, Chagnon U.S. Pat. No. 4,356,098 and Wyman U.S. Pat. No. 4,430,239 disclose ferrofluids which are Newtonian liquids containing suspended, small magnetic particles which do not settle out under the influence of gravity and an external magnetic field.
U.S. Pat. Nos. 4,834,898 and 4,906,382 to Hwang provide reagents and methods for magnetizing nonmagnetic materials utilizing a reagent comprising water containing particles of a magnetic material, such as magnetite, each of which is coated with a two layer surfactant coating including an inner layer and an outer layer. The inner layer covers the particle and is a monomolecular layer of a first water soluble, organic, heteropolar surfactant containing at least 3 carbon atoms and having a functional group on one end which forms a bond with the magnetic particle and a hydrophobic end. The outer layer coats the inner layer and is a monomolecular layer of a second water soluble, organic, heteropolar surfactant containing at least 3 carbon atoms and having a hydrophobic end bonded to the hydrophobic end of the first surfactant and a functional group oriented outwardly toward the water. Nonmagnetic particles are magnetized by contacting their surfaces with the magnetizing reagent in an aqueous medium and the coated magnetic particles couple with the nonmagnetic particles by adsorption. U.S. Pat. Nos. 4,087,004 and 4,125,400 to Nott et al., provide magnetic beneficiation of clays utilizing magnetic particulates, including particulates of ferroso-ferric oxide particles.
The present invention provides a method that allows improved processing of lower brightness clays by increasing the removal of iron- and titania-containing impurities in the clay. Since the selective mining of higher brightness crude clays continues to deplete the quantity of available reserves of these high brightness crudes, such method is of great benefit.